Obama is exercising "executive power" much as the Jacobite kings exercised "royal prerogative."  Goes to show that it is impossible to clearly separate executive from legislative power, and as I see the dispute in the States on this issue, it might be that Congress is the one interfering with the president's power make executive decisions.  If the Supreme Court were not today such a political institution we might hope for a clear line that all would accept: I don't think now that is possible.  In the end this is just one more example of the developing constitutional crisis in the States resulting from its having such an obsolete basis of government.

I would add that traditionally amnesties have always been an executive prerogative.